The invention relates to a method for manufacturing alkali metal or alkaline-earth metal formate, starting from a formate anion.
At the present, for example potassium formate is normally manufactured as a neutralization reaction between formic acid and potassium hydroxide, wherein formic acid must always be available. Also a direct synthesis between CO and KOH is a possible manufacturing technique for obtaining potassium formate directly (Encyclopedia of Chemical Technology, 3rd ed., vol. 18, p. 938). This latter reaction requires the application of a high temperature and pressure.
It is characteristic in all these methods that the raw material is potassium hydroxide, and when potassium formate is manufactured in a neutralization reaction, formic acid must always be used as a source of formate anion.
Sodium formate is obtained as a side product of pentaerythritol. Pentaerythritol is manufactured in a reaction between formaldehyde and acetaldehyde by using sodium hydroxide as a catalyst. A side product of this process is dilute sodium formate which can be further converted to formic acid with sulphuric acid.
In connection with this process, also electrodialytic manufacture of potassium formate from K2SO4 and HCOONa has been developed. An electrodialytic process with a concentrated sodium formate solution and a potassium sulphate solution as starting materials is described in international publication WO 96/01250. The electrodialytic manufacturing process starting from said materials requires various types of selective membranes. Furthermore, the electrodialytic device also requires much maintenance for securing its faultless operation, e.g. to prevent clogging of the membranes.
Swiss patent 439249, to which corresponds British patent 1033030, discloses a method for manufacturing inorganic and organic salts. Table 1 and example 5 of the patent present the conversion of formic acid into sodium formate with a weak-base anion exchanger in liquid form by mixing an aqueous solution of formic acid and sodium chloride with the weak-base anion exchanger dissolved in an organic phase. The manufacturing method requires the dissolving of the anion exchanger in a suitable organic solvent which is insoluble in water, mixing of the substances and separating the water phase for separating the final product.
Methods for manufacturing potassium formate, in turn, are disclosed in U.S. Pat. No. 2,913,318 and DE application publication 4126730. The methods are based on reactions between calcium hydroxide and carbon monoxide and between calcium hydroxide and formaldehyde, respectively. The former method requires a pressure reactor, and the latter requires handling of formaldehyde.
It is an aim of the invention to eliminate the above-mentioned draw-backs and to present a simpler manufacturing method which is suitable for industrial use and in which it is possible to start from a suitable formate solution or formic acid, ie. in principle from any substance containing the formate anion, and to obtain an alkali metal or alkaline-earth metal formate as a final product.
A simple way of manufacturing potassium formate was surprisingly found when studying anion exchange with solid ion exchangers between potassium chloride and sodium formate, wherein the ion exchange produced a potassium formate solution from a sodium formate solution or even from formic acid. In a corresponding way, a possibility was found to produce a calcium formate solution from a sodium formate solution or from formic acid.
Anion exchange processes have been primarily used to purify substances, and examples that can be mentioned include for example the removal of nitrate from drinking water, which is described in U.S. Pat. No. 4,479,877. It is also known to use a weakly basic anion exchanger to remove formic acid in the manufacture of pure formalin (J. T. McNulty, xe2x80x9cThe many faces of ion-exchange resinsxe2x80x9d, Chemical Engineering, June 1997, p. 99). The manufacture of alkali metal formate or alkaline-earth metal formate by anion exchange with a solid anion exchanger has not been previously disclosed.
In the following, the invention will be described in more detail with reference to the appended drawings which illustrate the steps of the anion exchange process under different conditions.
According to the invention, a solution containing formate ions is supplied to a solid anion exchange material, wherein they replace the anions bound to the material previously, e.g. in regeneration. Alkali metal or alkaline-earth metal formate can be eluted from the material by feeding an alkali metal or alkaline-earth metal salt of the anion which replaces the formate anion to the material. The selectivity of the anions used in the process towards the anion exchange material decreases in the following order:
A greater than HCOOxe2x88x92 greater than D,
in which D is an anion which is replaced with formate at the first stage, and A is an anion whose alkali metal or alkaline-earth metal salt is supplied to elute alkali metal or alkaline-earth metal formate, respectively, from the anion exchanger. For example, the order of selectivity for a (strong-base) anion exchanger is the following (Helfferich, F., Ion Exchange, p. 168):
NO3xe2x88x92 greater than Ixe2x88x92 greater than Brxe2x88x92 greater than Clxe2x88x92 greater than HCOOxe2x88x92 greater than CH3COOxe2x88x92 greater than OHxe2x88x92 greater than Fxe2x88x92
In the process, the anions passing through the anion exchange material are underlined. The anion exchanger cannot be directly converted from the Clxe2x88x92 form back to the formate form because of unadvantageous selectivity, but it must first be converted to OHxe2x88x92 form in a separate regeneration step.
The following is a description of a method according to a preferred embodiment step by step, relating to the manufacture of potassium formate.
An anion exchange, which contains Cl ions as a result of elution of potassium formate out in a preceding step, is first converted to OHxe2x88x92 form with NaOH:
Rxe2x80x94Cl+NaOHRxe2x80x94OH+NaClxe2x80x83xe2x80x83(1)
With weak-base anion exchangers, it is also possible to use NH4OH as the hydroxide source.
After this, the formate ions are bound from a sodium formate solution to an ion exchange bed which contains anion exchange material. Instead of a sodium formate solution, it is possible to use a formic acid solution. It is also possible to use a mixture of formic acid and sodium formate. Similarly, it is possible to use any substance whose aqueous solution contains formate ions and whose aqueous solution is not too alkaline, if a weak-base anion exchanger is used.
Rxe2x80x94OH+HCOONaRxe2x80x94HCOO+NaOH, or
Rxe2x80x94OH+HCOOHRxe2x80x94HCOO+H2Oxe2x80x83xe2x80x83(2)
By changing the anion exchanger back to Clxe2x88x92 form, a KCl solution can be used to elute potassium formate out of the anion exchange material:
Rxe2x80x94HCOO+KClRxe2x80x94Cl+HCOOKxe2x80x83xe2x80x83(3)
After this, the same anion exchange material can be used again by replacing chloride with hydroxide according to step (1). Sodium hydroxide eluted out in step (2) can thus be recirculated to step (1). After each step, washing with water is performed. The washing can be intensified by raising the temperature.
In different steps of the process, solutions are used whose concentration is in the range from 0.1 to 5 M. Also, in the elution of potassium formate from the anion exchanger, the concentration of potassium salt solutions used can be in the range from 0.1 to 5 M.
Ion exchange for producing potassium formate can take place in a temperature range from 0 to 110xc2x0 C., typically from 20 to 60xc2x0C.
In a way analogical to that presented above, it is possible to prepare calcium formate. The reactions (1) and (2) take place as above, but in the reaction (3), the anion exchanger is converted to chloride form with a calcium chloride solution:
2Rxe2x80x94HCOO+CaCl22Rxe2x80x94Cl+(HCOO)2Ca
According to an advantageous alternative, the manufacturing process, in which the same anion exchanger is used several times, is as follows:
Use is made of an anion A, whose selectivity to the anion exchange material is higher than that of formate, and an anion D, whose selectivity to the anion exchange material is lower than that of formate.
Use is made of a starting agent containing formic acid and/or a formic acid salt whose cation (starting agent cation) is different from the cation of the alkali metal or alkaline-earth metal formate (product cation).
In the first step, the anion exchanger is regenerated by feeding into it a solution that contains the anion D, wherein the anion A left therein at the production stage can be eluted from the anion exchanger.
In the second step, a solution that contains the formate anion, such as formic acid and/or a formic acid salt whose cation is the starting agent cation, is fed into the anion exchanger.
In the third step, i.e. the production step, a solution which contains the anion A and in which the cation is the product cation, is fed into the anion exchanger.
All the steps are implemented in such a way that the solution is fed in a continuous flow through the anion exchange material, and the anion exchange between the solid anion exchanger and the solution takes place during this flow. The anion exchanger is preferably in aflow-through column, through which the solution is led. The solutions to be led in subsequent steps through the anion exchanger are preferably aqueous solutions.
These steps can be repeated several times in succession for the same anion exchanger. Furthermore, it is possible to circulate the anion D eluted in the second step to the first step, i.e. to the regeneration step. If in the second step the feeding solution contains a formate whose cation is the starting agent cation, a regeneration solution is obtained which contains the anion D and the starting agent cation.
Strong-base anion exchangers act in the pH range from 0 to 14, whereas the active range of weak-base anion exchangers is in the pH range from 0 to 9. Consequently, when selecting the formate ion source, one should take into account the strength of the ion exchange resin and the resulting active pH range. In a strong-base anion exchanger, the functional group is generally a quaternary ammonium ion, which is trimethylamine in type I and dimethyl-xcex2-hydroxyethylamine in type II presented below. 
Type I is slightly more basic than type II, i.e. the anions are slightly more firmly bonded to the anion exchanger of type I. Consequently, the regeneration of type II to the OHxe2x88x92 form is slightly easier. In type II, the chemical stability and temperature resistance is slightly lower than in type I.
In a weak-base anion exchanger, the functional group is generally a tertiary aminexe2x80x94N(CH3)2.
Solid ion exchangers exist in various forms, having different porous structures and being based on different polymers. This invention is not dependent on the form, porous structure or polymer. The shape of the ion exchanger can be e.g. sphere, powder, fibre, or flake. Macroporous or gel-like ion exchanger resins work.
The invention will be described in the following examples which do not restrict the invention. The formate content is determined by ion chromatography according to the standard SFS-EN ISO 10304-01, and the chloride content by a chloride-selective electrode. The volume of the eluted solution is given in bed volumes BV, which in the tests was 100 ml or 200 ml. The water used, in which also the solutions were prepared, was ion-exchanged water whose conductivity was less than 3 xcexcS/cm.